What are hydraulics and pneumatics?
A fluid is something that flows: a liquid or a gas. Both systems use a fluid inside closed pipes to carry force from one place to another.
- Hydraulics uses a liquid, usually oil. The pressure is high (often 100 to 300 bar).
- Pneumatics uses a gas, usually compressed air. The pressure is lower (often 6 to 10 bar).
Pressure means force on each unit of area: p = F / A. One pascal (Pa) is 1 N on 1 m². One bar is 100 000 Pa.
The physics behind them
Pascal's law: pressure put on a closed fluid goes to every part of the fluid equally. So if the small piston makes pressure p, the big piston feels the same p.
Force out: F2 = p × A2. With the same p, a piston with 25 times more area gives 25 times more force. This is called force multiplication.
But nothing is free. The liquid volume stays the same, so A1 × d1 = A2 × d2. The big piston moves 25 times less distance. Work in = work out (if we ignore friction).
Compressibility: a liquid hardly changes volume when pushed, so it moves loads exactly and stays still when you stop. Air shrinks when squeezed (Boyle's law: p × V is about constant), so it acts like a spring.
Main components
Every system has four jobs:
- Make pressure: a hydraulic pump (gear, vane or piston type) or a pneumatic compressor.
- Store and clean: an oil tank with a filter, or an air tank with a dryer. Air must be dry and clean.
- Control: valves. A direction valve picks the path, a pressure relief valve opens when pressure is too high (safety), a flow valve sets the speed, a check valve lets fluid go one way only.
- Do the work: an actuator. A cylinder gives a straight push; a motor gives turning.
A single-acting cylinder gets fluid on one side and a spring brings it back. A double-acting cylinder gets fluid on both sides, so it pushes out and pulls in.
Force of a cylinder going out: F = p × A. Coming in, the rod takes up some area, so the force is a little smaller.
Typical circuit diagrams
Engineers draw circuits with standard symbols, not pictures. The symbols are the same in many countries (ISO 1219). Read a circuit from the power source to the actuator.
- Circle with a triangle: pump or compressor (the triangle points the way the fluid goes).
- Square boxes side by side: a valve. Each box is one position. Arrows inside show the paths. A spring or a lever symbol at the side shows how it is moved.
- Rectangle with a rod: the cylinder.
- Lines: pipes. Solid lines carry working fluid; dashed lines carry a control signal.
Simple pneumatic circuit: compressor, air tank, 5/2 valve (5 ports, 2 positions), double-acting cylinder. Press the lever: air goes to the back of the cylinder, the rod goes out. Release: the spring flips the valve, air goes to the front, the rod comes in.
Simple hydraulic circuit: tank, pump, relief valve, direction valve, cylinder, and a return line back to the tank. The oil is used again and again.
Hydraulics or pneumatics: which one?
| Hydraulics | Pneumatics | |
|---|---|---|
| Fluid | Oil | Air |
| Pressure | High | Low |
| Force | Very large | Small to medium |
| Position | Stops exactly | Springy, less exact |
| Speed | Slower | Fast |
| Cleanliness | Oil can leak | Clean; air just escapes |
| Examples | Excavator, press, brake, lift | Bus door, packing machine, dentist drill |
Designing and simulating a circuit
To design a circuit, follow these steps:
- Write what must happen: push out, hold, come back. How big is the force? How far? How fast?
- Find the force needed, and choose a pressure. Then the area is A = F / p. Pick the next standard cylinder size, with some extra for friction (about 10 to 25 percent).
- Choose the valve: two positions for out and in; a third position to hold in the middle.
- Add a relief valve (hydraulics) or a pressure regulator (pneumatics) for safety.
- Draw it with standard symbols. Number each part.
- Test it in a free simulator before building it.
Always check: what happens if power fails? Safe designs go to a safe position by themselves.
Key formulas and definitions
- Pressure: p = F / A (1 Pa = 1 N/m², 1 bar = 100 000 Pa)
- Pascal's law: p1 = p2, so F1 / A1 = F2 / A2
- Force multiplication: F2 = F1 × (A2 / A1)
- Volume stays the same: A1 × d1 = A2 × d2
- Cylinder force: F = p × A, with A = π d² / 4
- Flow and speed: Q = A × v (v = Q / A)
Worked examples
1. A hydraulic jack has a small piston of area 5 cm² and a big piston of area 200 cm². A force of 50 N pushes the small piston. Find the force on the big piston.
Step 1: pressure p = F1 / A1 = 50 / 5 = 10 N/cm². Step 2: the same pressure acts on the big piston: F2 = p × A2 = 10 × 200 = 2000 N.
2. In the jack above, the small piston goes down 30 cm. How far does the big piston rise?
A1 × d1 = A2 × d2, so d2 = 5 × 30 / 200 = 0.75 cm. A force 40 times bigger, a distance 40 times smaller.
3. A car brake pedal pushes a master piston of area 2 cm² with 100 N. The brake piston at the wheel has area 10 cm². What force acts at the wheel?
p = 100 / 2 = 50 N/cm². F2 = 50 × 10 = 500 N. The brake force is 5 times the pedal force.
4. A garage lift must hold a car that weighs 12 000 N on a big piston of area 600 cm². The small piston has area 20 cm². What force must the worker put?
F1 = F2 × A1 / A2 = 12 000 × 20 / 600 = 400 N. (A person can easily push 400 N with a lever.)
5. A pneumatic cylinder has a piston of diameter 5 cm and works at 6 bar. Find the force when it goes out.
Step 1: p = 6 bar = 6 × 10⁵ Pa. Step 2: A = π d² / 4 = π × (0.05)² / 4 = 1.96 × 10⁻³ m². Step 3: F = p × A = 6 × 10⁵ × 1.96 × 10⁻³ ≈ 1178 N, about 1.2 kN.
6. A hydraulic press needs a force of 20 kN. The pump gives 100 bar. What piston diameter is needed? (Ignore friction.)
p = 100 bar = 10⁷ Pa. A = F / p = 20 000 / 10⁷ = 2 × 10⁻³ m². d = √(4A / π) = √(4 × 2 × 10⁻³ / π) = √(2.55 × 10⁻³) ≈ 0.0505 m, so about 5 cm (choose the next standard size, 50 or 63 mm).
Common mistakes
- Thinking the machine makes free energy. Force grows, but distance shrinks by the same factor, so work in = work out.
- Mixing units. Use N and m² to get Pa, or N and cm² to get N/cm². 1 bar = 10 N/cm².
- Using diameter instead of area. A = π d² / 4. Double the diameter makes 4 times the area.
- Saying air is as stiff as oil. Air can be squeezed, so pneumatic positions are not exact and the speed can change with load.